My NC21 leaked at the fuel tap when I got it, and I have happened across a few posts that seem to indicate others have had the same problem. I tried all the wrong ways to solve this problem, but finally found my problem. The diaphram inside my fuel valve was assembled wrong.

I happened across this video for replacing the internal fuel valve bits for a Honda GL1500.

https://www.youtube.com/watch?v=2APRgiXcuGU

Apparently some Goldwings used a very similar system as the NC21. In any case, watch the Youtube video and make sure you are pulling the smaller diaphram through the middle fuel tap piece. I wasn't and it created all sorts of trouble. I thought an O-ring was missing (and so did someone else in another thread). I put an O-ring in, and it started sending fuel down the vacuum line. I capped the vacuum port and connected the vacuum line to the fuel vent hole. That seemed to work, but I wanted it set up correctly. That's when my googling got me to the Youtube video.

I did order the repair kit for the GL1500 and it "looks" like it would work on the NC21. Mine is working now, so I don't think I'll take it apart to check any time soon.